1. Field of the Invention
The present invention relates to a programmable logic array and more particularly to a field programmable logic array (FPLA).
2. Description of the Prior Art
An FPLA is a small-quantity large-variety IC (LSI) because it is programmable in the field. A structure of the FPLA is shown in FIG. 1. Inputs I and signals from internal feedback JK-flip-flops 7 (JKFF's) are supplied to an input buffer 3. Outputs from the input buffer 3 are decoded by a 2-bit decoder 4 into each pair of the inputs I and the JKFF's 7. Outputs of the 2-bit decoder 4 are supplied to an AND array 1 as inquiry signals. The AND array 1 can connect any inputs by a field-writable blown-junction type program method. Outputs of the AND array 1 which reflect the inquiry signals are supplied to an OR array 2 through an OR array driver 5. The OR array 2 can also connect any inputs by the blown-junction type program method. Outputs of the OR array 2 are supplied to an OR output buffer 6. Outputs of the OR output buffer 6 are supplied to the internal feedback JKFF's 7 and output latch 8. The inputs to the JKFF's 7 are latched by a clock CK1 and are resettable. The inputs to the output latch 8 are latched by a clock CKO.
FIG. 2 shows details of the AND array 1 and the OR array 2 of the blown-junction type program method. The inquiry signals from the decoder 4 are applied to input lines 10 of the AND array 1, and output signals are produced on output lines 11 in accordance with the inquiry signals. The output lines 11 are connected to input lines 12 of the OR array 2 through the OR array drivers 5 and output signals are produced on output lines 13 of OR array 2 in accordance with the inquiry signals. As shown in FIG. 2, the input lines 10 and the output lines 11 of the AND array 1, and the input lines 12 and the output lines 13 of the OR array 2 are connected by transistors. FIG. 3 shows circuits before and after programming by the blown-junction type programming method. As seen from FIG. 3, the transistor functions as a diode which electrically connects the input line and the output line after it has been programmed. The respective columms (corresponding to the output lines 11) of the AND array 1 are called AND terms and the number of AND terms represents the number of programs of the AND array 1. The output lines 11 of the AND array 1 and the input lines 12 of the OR array 2 are collectively called AND term lines.
The FPLA has an advantage of field programming capability. However, there may be defects which can be detected only after it has been programmed in the field. For example, there may exist a wrong bit which cannot be programmed because of manufacturing defects. If an error is detected by a check after the programming, the FPLA cannot be used and it must be thrown away. Even when all of the terms of the AND array 1 are not used, the FLPA must be thrown away.